Python Basics — Format Specifiers

Format Specifiers

Format specifiers control how values are displayed — width, alignment, decimal places, number bases, and more. They work with both f-strings and str.format().

The Format Spec Mini-Language

The general syntax inside the braces is:

{value:[[fill]align][sign][#][0][width][grouping][.precision][type]}

Don’t worry about memorizing this — let’s break it down with examples.

Width and Alignment

Syntax	Meaning	Example	Result
{:10}	Min width 10	f"{'hi':10}"	"hi "
{:<10}	Left-align	f"{'hi':<10}"	"hi "
{:>10}	Right-align	f"{'hi':>10}"	" hi"
{:^10}	Center	f"{'hi':^10}"	" hi "

Fill Characters

# Default fill is space
print(f"{'Title':^20}")        #         Title

# Custom fill character
print(f"{'Title':*^20}")       # *******Title********
print(f"{'Title':=^20}")       # =======Title========
print(f"{'Title':-^20}")       # -------Title--------
print(f"{'Title':.^20}")       # .......Title........

Number Formatting

Decimal Places

pi = 3.14159265

print(f"{pi:.0f}")   # 3
print(f"{pi:.1f}")   # 3.1
print(f"{pi:.2f}")   # 3.14
print(f"{pi:.4f}")   # 3.1416
print(f"{pi:.10f}")  # 3.1415926500

Thousands Separator

big = 1234567890

print(f"{big:,}")       # 1,234,567,890
print(f"{big:_}")       # 1_234_567_890

# Combining with decimals
price = 1234567.89
print(f"${price:,.2f}")  # $1,234,567.89

Percentage

rate = 0.856
print(f"{rate:%}")       # 85.600000%
print(f"{rate:.1%}")     # 85.6%
print(f"{rate:.0%}")     # 86%

Sign Display

pos = 42
neg = -42

print(f"{pos:+}")    # +42  (always show sign)
print(f"{neg:+}")    # -42
print(f"{pos:-}")    # 42   (only show negative sign — default)
print(f"{pos: }")    # ' 42' (space for positive, minus for negative)
print(f"{neg: }")    # -42

Number Bases

Type	Meaning	Example	Result
b	Binary	f"{42:b}"	"101010"
o	Octal	f"{42:o}"	"52"
x	Hex (lower)	f"{255:x}"	"ff"
X	Hex (upper)	f"{255:X}"	"FF"
d	Decimal	f"{42:d}"	"42"

num = 255

print(f"Decimal:  {num:d}")    # Decimal:  255
print(f"Binary:   {num:b}")    # Binary:   11111111
print(f"Octal:    {num:o}")    # Octal:    377
print(f"Hex:      {num:x}")    # Hex:      ff
print(f"Hex:      {num:X}")    # Hex:      FF

# With prefix (#)
print(f"Binary:   {num:#b}")   # Binary:   0b11111111
print(f"Octal:    {num:#o}")   # Octal:    0o377
print(f"Hex:      {num:#x}")   # Hex:      0xff

Zero Padding

# Pad numbers with zeros
print(f"{42:05}")      # 00042
print(f"{42:08}")      # 00000042
print(f"{3.14:08.2f}") # 00003.14

# Useful for file names, IDs
for i in range(1, 4):
    print(f"file_{i:03}.txt")
# file_001.txt
# file_002.txt
# file_003.txt

Scientific Notation

big = 1234567890.0
small = 0.000123

print(f"{big:e}")      # 1.234568e+09
print(f"{big:.2e}")    # 1.23e+09
print(f"{small:e}")    # 1.230000e-04
print(f"{small:.2e}")  # 1.23e-04

# General format — chooses fixed or scientific
print(f"{big:g}")      # 1.23457e+09
print(f"{small:g}")    # 0.000123
print(f"{3.14:g}")     # 3.14

Building a Formatted Table

# Complete table example
data = [
    ("Alice", 95, 3.87),
    ("Bob", 87, 3.45),
    ("Charlie", 92, 3.72),
]

print(f"{'Name':<10} {'Score':>6} {'GPA':>6}")
print(f"{'-'*10} {'-'*6} {'-'*6}")
for name, score, gpa in data:
    print(f"{name:<10} {score:>6} {gpa:>6.2f}")

Output:

Name        Score    GPA
---------- ------ ------
Alice          95   3.87
Bob            87   3.45
Charlie        92   3.72

Quick Reference

Format	Description	Input	Output
:.2f	2 decimal places	3.14159	3.14
:,	Thousands separator	1000000	1,000,000
:.1%	Percentage (1 dec)	0.856	85.6%
:05	Zero-padded width 5	42	00042
:>10	Right-align width 10	"hi"	" hi"
:<10	Left-align width 10	"hi"	"hi "
:^10	Center width 10	"hi"	" hi "
:+	Always show sign	42	+42
:b	Binary	10	1010
:x	Hexadecimal	255	ff
:.2e	Scientific notation	1234.5	1.23e+03

Summary

• Format specifiers control how values are displayed in f-strings
• Use :.Nf for decimal places, :, for thousands separators
• Use <, >, ^ for alignment with optional fill characters
• Use :.N% for percentage formatting
• Use :b, :o, :x for binary, octal, and hex
• Use :0N for zero-padding
• Combine specifiers: f"${price:>12,.2f}"

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